Modern oil well drilling technology and techniques have opened up new oil reserves that had previously been unreachable. Technology such as improved seismic imaging identifies new reserves. Techniques such as horizontal drilling may allow these identified reserves to be reached by a well bore. Technology and know-how combine to provide a variety a non-vertical drilling approaches to reach new reserves.
These new approaches, however, have also presented new problems. One of the most significant problems is failure of the drill bit to penetrate a particular formation, resulting in stuck pipe or other drilling stoppages. Excessive friction between the well bore and the drill or pipe is the usual culprit in such drilling failures.
Drilling and completion engineers continually face the problem of overcoming resistances to drilling the wellbore or placing production equipment into a newly completed well. Most frictional forces encountered in drilling are due to hole conditions and geometry.
No hole is truly vertical. Drill pipe, casing, screens, and the like, will come in contact with the wellbore at numerous points. The problem is exacerbated in deviated wellbores. Modern wellbores may be slanted, horizontal, or may even turn back toward the surface. Frictional resistance is commonly referred to as drag in the context of a non-vertical wellbore.
Drill lubricating fluids, also known as muds, are commonly supplied to a drill to reduce the coefficient of friction where the drill, pipe, or equipment contacts the formation being drilled. By way of example and not limitation, muds include chemicals, fluids, and fluid systems such as oil mud, parafin oil, olefin oil solutions, water-based mud, synthetic systems, blendes, calcium chloride brine, emulsified or ground gilsonite, straight chain normal hydrocarbons. A mud may also comprise a collection of products.
The lubricity of a mud is important for enhancing the economics of drilling and completing high angle holes. Lubricity is a measure of the coefficient of friction between a moving part and a surface in contact with the part. The lower the coefficient of friction, the greater the lubricity. The coefficient of friction, u, is defined as the ratio of the force, F, required to move an object in contact with a surface to the force, W.sub.1, pushing downward or perpendicular to the object: u=F/W.sub.1. The coefficient of friction may alternatively be called the friction coefficient, friction factor, or the lubricity coefficient.
The lubricity of a mud is a measure of the mud's ability to lower torque and drag forces. Muds are frequently tested in the laboratory to obtain a rough estimate of the mud's lubricity before the mud is used in the field. It is important for the economics of drilling, and to keep the drilling operation out of trouble, that the laboratory measurements of lubricity reliably correlate to the lubricity observed in the field. Drill operators depend on reliable lubricity coefficients as input into computer drilling models that predict drillstring loads to optimize casing runs. The friction coefficient is used in these models prior to drilling the well to enhance the well design with respect to torque and drag. Drilling fluid (mud) companies depend on reliable friction coefficients to recommend proper mud systems, determine the optimum lubricant amounts, and to develop new lubricant additives.
A variety of lubricity measuring machines have been developed by the oil industry. The earliest machines were adapted from other industries.
To evaluate lubricants for torque reduction, an early device, the Timken lubricity tester, was adapted from an industry standard extreme pressure (EP) torque measuring instrument. This early apparatus was used to determine the friction coefficient under high loads and extremes pressures, such as observed in bearings and engines.
The first mud lubricity tester was modified from the Timken apparatus, and was used to identify extreme pressure lubricants to extend the life of bit bearings. The advent of sealed bearing bits has rendered these lubricants obsolete.
The Fann instrument was developed to test lubricity under substantially vertical conditions. It uses a steel block to simulate the wall of a hole. The block is pressed against a rotating steel ring by a torque measuring arm. The coefficient of friction is determined by the amount of current required to drive the rotating ring at a given rpm while the steel block is immersed in the mud. The current drawn by the device is converted to a lubricity coefficient by a previously calibrated chart. The Fann device tests lubricants under standard conditions to see how they compare under those conditions.
Another lubricity tester is the Lubricity Evaluation Monitor (LEM). The LEM differs from the Fann machine in that the mud is continuously circulated across the test surfaces, and it can use a sandstone core, shale pieces, filter cakes or steel pipe as a test medium. A stainless steel shaft rotates under an applied load. The load forces the shaft into the test medium, and the torque required to turn the shaft is plotted against time. The LEM tests lubricants under a variety of conditions.
Yet another lubricity tester is the HPHT lubricity tester (HLT). The HLT was developed to address problems with previous testers. One problem with the Fann and LEM machines is that the coefficients derived are not scalable to actual downhole conditions. This is particularly true with respect to temperatures and pressures encountered downhole. The HLT was developed in an effort to more accurately simulate downhole conditions.
The primary components of the HLT consist of a friction mechanism contained in a test cell, a lathe frame onto which the test cell is mounted, mud circulation and fluid injection means (separately housed), and a computerized control, monitoring and display system.
HLT, like the Fann and LEM devices, is best suited for simulating substantially vertical drilling conditions. Furthermore, these devices essential model static drilling conditions which may not accurately simulate the dynamic nature of actually drilling.
Non-vertical, deviated, and dynamic conditions, however, present a different set of torque and resistance problems than those modeled by the prior art. For example, drag, the resistance to drilling encountered by deviated drilling conditions, is not well modeled by the above devices.
A particular problem in modeling a horizontal drilling operation is that typical models for conventional (vertical) drilling use rotation to generate data, but horizontal drilling operations more often use a mud motor and do not rotate the drill string. Horizontal sliding forces are not the focus of prior art devices.
Lubricants that are successful under vertical conditions may not be successful under deviated conditions.
To effectively test lubricants under dynamic field circumstances, a portable lubricity evaluation device that could be transported to the drill site for testing muds under the precise conditions being encountered in the field would provide great benefits for drilling success, economies, and efficiency.
Absent an on-site lubricity testing apparatus, core samples, filter cake, and other indicia of drilling conditions, must be sent to the nearest laboratory to develop an optimal fluid in response to the indicia. The mud, or instructions for its formulation, must then be sent to the field for implementation.
Remote mud analysis means delays of hours or days in drilling, and is subject to miscommunication or delayed communication about the drilling operation. A portable lubricity evaluation device that could be brought to the drilling site would obviate these problems.
The value of identifying the optimal drilling mud is presented in the following example.
A drilling program in south Texas was initiated on the basis of 3-dimensional seismic data that revealed untested deep zones in six wells beneath the depth of 12500 feet where production had stopped. Among the objectives of the program was to reach the untested reserves below 12500 feet and to optimize drilling efficiency. During the course of the program, the selection of drilling fluids became the central issue in optimizing drilling efficiency.
At the beginning of the project, it was not anticipated that stuck pipe would be a major problem, because the field, which had produced for twenty years, did not have a history of stuck pipe. The first three wells that were drilled, however, all failed as a result of stuck pipe. The abundance of stuck pipe problems revealed that there were multiple pressure depleted zones at the depth of interest. As a result of the failure of the first three wells, the selection of drilling fluid came under increased scrutiny.
The choice of drilling fluids was amended for the remaining three wells of the program. Oil based muds were eliminated from consideration, and an economical water based mud was specified. Additives that were readily available in the field were used. Ultimately, a bentonite based drilling fluid with an EP lubricant, gilsonite, and micronized cellulose fiber additive was selected.
The results in the last three wells were dramatically different from the experience of the first three wells. Each of the last three wells was a success. The mud was modified as needed based on the condition of the filter cake and drilling conditions.
The program described above took advantage of a portable dynamic filtration apparatus to evaluate the filter cake of the drill operation on-site. Muds were designed on the basis of filter cake results. The program did not, however, have use of a portable lubricity evaluation device of the present invention. The example is provided to illustrate the impact that the drilling fluid can have on the success of a well. It will be understood by those skilled in the art that access to a lubricity testing apparatus on-site would have enhanced the efficiency of the program described above.
Therefore, it is an object of the present invention to provide a lubricity evaluation device that simulates non-vertical drilling conditions to measure the effectiveness of a lubricant against drag and other frictional resistance forces encountered in non-vertical or deviated holes. The disclosed embodiment permits measurement of data that enables the calculation of a coefficient of friction for sliding without rotation.
It is also an object of the present invention to provide a lubricity evaluation device that can measure the lubricity of a fluid under a variety of load, torque, pressure, temperature, and frictional conditions.
It is a further object of the present invention to provide a lubricity evaluation device that can test a fluid under dynamic conditions encountered in the field.
It is another object of the invention to provide a portable lubricity evaluation device.